The present invention relates to a coaxial cavity resonator defined in the preamble of claim 1, which is particularly suitable for a structural part of a filter in radio devices.
Resonators are used as the main structural part in the manufacture of oscillators and filters. The important characteristics of resonators include, for example Q-value, size, mechanical stability, temperature and humidity stability and manufacturing costs.
The resonator constructions that are known so far include the following:
1) Resonators Compiled of Discrete Components, Such as Capacitors and Inductors
Resonators of this kind entail the drawback of internal dissipation of the components and therefore clearly lower Q-values compared to the other types.
2) Microstrip Resonators
A microstrip resonator is formed in the conductor areas on the surface of a circuit board, for example. The drawback is radiation dissipation caused by the open construction and thus relatively low Q-values.
3) Transmission Line Resonators
In a transmission line resonator, the oscillator consists of a certain length of a transmission line of a suitable type. When a twin cable or coaxial cable is used, the drawback is relatively high dissipation and a relatively poor stability. When a waveguide is used, stability can be improved, but the dissipation is still relatively high because of radiation when the end of the pipe is open. The construction can also be unpractical large. A closed, relatively short waveguide resonator is regarded as a cavity resonator, which is dealt with later.
4) Coaxial Cavity Resonators
Resonators of this type have a construction which is not merely a piece of coaxial cable but a unit which was originally intended as a resonator. It includes, among other things, an inner conductor and an outer conductor, which are air-insulated from each other, and a conductive cover, which is connected with the outer conductor. A relatively good result can be achieved by this construction. The length of the resonator is at least in the order of one fourth of the wavelength, xcex/4, of the variable field effective in it, which is a drawback when aiming at minimising the size. The width can be reduced by reducing the sides of the outer conductor and the diameter of the inner conductors. However, this leads to an increase of resistive dissipation. In addition, because of the reduction in the thickness of the construction, it may be necessary to support the inner conductor by a piece made of a dielectric material, which causes considerable extra dissipation in the form of dielectric loss and increases the manufacturing costs.
5) Helix-resonators
This type is a modification of a coaxial resonator, in which the cylindrical inner conductor is replaced by a helical conductor. Thus the size of the resonator is reduced, but the clearly increased dissipation is a drawback. Dissipation is due to the small diameter of the inner conductor.
6) Cavity Resonators
Resonators of this type are hollow pieces made of a conductive material, in which electromagnetic oscillation can be excited. The resonator can be rectangular, cylindrical or spherical in shape. Very low dissipation can be achieved with cavity resonators. However, their size is a drawback when the aim is to minimise the size of the construction.
7) Dielectric Resonators
Coaxial cables or a closed conducting surface is formed on the surface of the dielectric piece. The advantage is that the construction can be made in a small size. Relatively low dissipation can also be achieved. On the other hand, dielectric resonators have the drawback of relatively high manufacturing costs.
8) Hat Resonators
A subclass of coaxial cavity resonators, here called hat resonators are well described as prior art in U.S. Pat. No. 4,292,610 by Makimoto, see FIG. 1. This type of resonator is a coaxial cavity resonator, as described above, with an additional disc on the open end of the waveguide, having a larger diameter than the waveguide. The advantage is that the construction can be made in a small size. Relatively low dissipation can also be achieved. The surface area of the disc and distances to the walls of the resonator are dimensioned so that due to the extra capacitance created between the disc and the cavity, the resonator can be made substantially smaller.
A further development of the hat resonator is described in U.S. Pat. No. 3,496,498 by Kawahashi et al and in JP 57-136804 A by Mitsubishi, where a multiple of discs, or grooves, are arranged on a resonator rod along the whole length of the resonator rod. By increasing the capacitive coupling to the cavity wall, the physical length of the conductive body may be reduced. A drawback with this type of multiple disc resonator is that the Q-value of the resonator decreases compared to the Q-value of the hat resonator.
Another coaxial cavity resonator having a reduced size is described in U.S. Pat. No. 3,448,412 by E. C. Johnson, where the conductive body and the interior of the cavity have intermeshing concentric tubular members that simulates a folded coaxial line. The size of the resonator is further reduced by a capacitive coupling between the top part of the conductive body and the interior of the housing, such as a hat resonator described above. Although this resonator has a reduced size it is still fairly large in size and will have the drawback of being mechanical unstable if the volume of the cavity is reduced, due to the design of the conductive body.
The object with the present invention is to provide a coaxial cavity resonator having a small size, good mechanical stability and a high Q-value compared to the above mentioned prior art.
A coaxial cavity resonator, that is an elaborate hat resonator, according to the invention is characterised in what is set forth in the independent claim. Some preferred embodiments of the invention are set forth in the dependent claims.
The basic idea of the invention is the following: The construction is a coaxial cavity resonator comprising at least one conductive body, which body is open at one end and shortened from a quarter-wave resonator. The conductive body includes a main rod, which is in one end attached to the cavity wall, and a main disc attached to the free end of the main rod. The cavity further comprise one or more conductive plates located between the main disc and the side walls, at the first side of, and out of galvanic contact with, the main disc, to create extra capacitive couplings between the main disc and the cavity walls via the plate(s). Additional discs may also be attached to the main rod. The shortening is carried out by creating air-insulated extra capacitance between the resonator cavity walls via the conductive plates and a mechanical structure at the open end of the conductive body.
The invention has the advantage that because of the manner of increasing the capacitance, the resonator can be made substantially smaller than a prior art quarter-wave resonator, which has the same Q-value. The improvement achieved can also be used partly for saving space and partly for maintaining a high Q-value compared to the Q-value for a resonator with a single top capacitance, such as a tuning screw.
Furthermore, a smaller resonator according to the present invention has the advantage to allow the volume of the cavity to be substantially smaller for a specific frequency, compared to prior art solutions.
In addition, the invention has the advantage that when the resonator is shortened it becomes mechanically stronger and therefore also more stable with regard to its electrical properties. Support pieces that increase the dissipation are not needed in it, either.
In the following, the invention will be described in more detail with reference to the accompanying drawings.